Why not - I used to add my previous gaming machines to my homelab multiple times. The only problems are cheap mainboards. Lenovos Legion Rebuilds I don't know enough about to say if their hardware is ok for that - their workstations would better be suited, I gues. But usually I'd say: why not. Just remeber to have regular backups in place, just in case
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A) don't buy TVs - big monitors/bfgd ir public displays B) log dns requests and IPs the tv wants to connect to. Forward that's requests to anything in your network (often it's just a get request or ping request to that destinations, that satisfies the tv)